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August 29, 2016

ZX Spectrum Harlequin Rev. G - S-Video Mod

After using my Harlequin a bit more, I decided I really needed to improve the video quality. The Composite output I was getting was just too crazy. Luckily, PokeMon over at the Sinclair ZX World forums has come up with an S-Video solution here that helped.

It uses the Mini DIN-8 connector, so most European users probably won't want to bother with this mod, since they can probably get superior RGB output using a modified SCART cable. Being located in the USA, this was the best video solution I could find that would allow me the option of Composite & S-Video without using using any converters/adapters. Just a handmade S-Video cable.

I read over his post/schematics thoroughly and realized I could modify his design a bit to make it a bit easier for me. Instead of running wire above and below the PCB and modding the R48 hole on the PCB, I figured out that I could just use 2 wires and 2 resistors and get the same results, running everything up top.

This is how I got S-Video output on my Harlequin Rev G. for people that have an interest in this:

Remove C35

Remove R49

Remove R8

Replace
D11 and R12 with 75R resistors(soldering only the top resistor pins to
the holes connected directly to pin 2-3 of the Mini Din)

Solder a wire from U48 pin 11 to the bottom resistor pin on D11

Solder a wire from the right side of C35 to the bottom resistor pin on R12

It's really that simple! But, I will include some photos of my endeavor that will hopefully clear up any questions you might have about it.

THE S-VIDEO MOD:

First I removed the Diode(D11), Resistors (R49, R12 & R8) and Capacitor C35.

Next, I snipped off the leads on the two resistors.

I cut two wires and soldered them to the short lead of the 75R resistors.

The end result looked like this.

After preparing the wiring, I soldered one of the resistor leads to the top hole of R12 and the other end to the right side hole of C35(leaving the bottom hole of R12 open). And I also soldered the second resistor lead to the top hole of D11 and the end of that wire to Pin 11 on U48(leaving the bottom hole of D11 open as well).

This probably sounds more complex than it is, so here are some photos:

That's how easy the mod is!

THE CABLE:

I didn't document the custom S-Video to Mini DIN-8 cable I made with photos very good. I just used an S-Video cable I had laying around, a pin out diagram I found online and used a DIY Mini DIN-8 plug like this for the other end:

I paid close attention to my wiring. Using a multimeter while I worked to make sure everything was being properly wired.

Here is the S-Video pin out diagram I used:

And here is one I made of the RGB Mini DIN-8 socket on the Harlequin:

CABLE WIRING:

Pin 3(Luma) on the S-Video end to Pin 2 of the Mini DIN-8
Pin 4(Chroma) on the S-Video end to Pin 3 of the Mini DIN-8
Pin 1(Ground Luma) and Pin 2(Ground Chroma) on the S-Video End to Pin 1 of the Mini DIN-8

My S-Video cable also had a grounding shield wire which I soldered onto the shielding of my Mini DIN-8 plug.

As you can see from the photo above, I also had to make a case cut to expose the Mini DIN-8 socket.

CASE:

To help with the case cut, I used a template shared by Ben Veersteg on his Facebook page(Thanks Ben!) and resized it to 74mm width. Printed that out and it aligns perfectly on the backside of my Spectrum case.

This is a very tricky part of the mod, since the plastic can be brittle. What I did was used a small drill hole for starting in the middle and then switched to a grinding bit on my Dremel for carving it out to size.

Just be very careful when cutting your case. Some people suggest doing more of a saw technique, but I preferred having it round and cut to fit. So that is why I chose the Dremel.

The Verdict:

After having done all of the above, I plugged my Spectrum into the S-Video slot on my TV and was greeted with a very stable image. No more wavy/rainbow/jagged lines craziness!

I am super stoked having done this! Finally, I have a solid ZX Spectrum capable of running a nice clear image in NTSC video land. And without extra converters and such!

In the videos above, you can watch as I switch between the Composite and S-Video outputs. Notice how much more clean and stable the S-Video looks on my CRT. It's great!

And here is a side-by-side comparison for your view pleasure:

In hindsight, I think it would have helped having done this mod during the whole building process instead of after. It was tricky getting some of the parts removed after it was fully assembled and I could have used a smaller gauge wire and maybe have laid them out a bit nicer. Ideally, I don't care for the way I laid the wire on top of the U48 for example. But... It works great and that's all that really matters to me.

I really hope this post helps anyone interested in modding their Harlequin's for S-Video output. I struggled finding information, so if you need any help, let me know.

I'd like to thank PokeMon on the Sinclair ZX World Forum for coming up with this design and sharing it with us all and to Ben Veersteg for his RGB cut-out template on Facebook.